The present invention relates to substantially nontoxic gas generating compositions which upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to high nitrogen gas generants that produce combustion products having not only acceptable toxicity levels, but that also exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.
Pyrotechnic gas generants incorporating an oxidizer such as potassium nitrate, potassium perchlorate, molybdenum disulfide, chromic chloride, copper oxide, or iron oxide with alkali metal and alkaline earth metal azides have been commercially successful. Sodium azide has been the most extensively used azide in solid gas generants for airbag systems as described in U.S. Pat. Nos. 2,981,616, 3,741,585, 3,865,660, 4,203,787, 4,547,235, and 4,758,287, the teachings of which are herein incorporated by reference.
However, azides are very toxic and sodium azide is a very poisonous material, both orally and dermatologically. In fact, sodium azide is shipped as a class B poison similar to other extremely toxic materials, such as sodium cyanide and strychnine. Sodium azide hydrolyzes, forming hydrazoic acid which is very poisonous and reacts with heavy metals such as copper and lead to form very sensitive covalent azides which are readily detonated by shock or impact. In addition, propellants prepared from sodium azide are not very efficient gas producers and result in gas outputs of only about 1.3 to 1.6 moles of gas per 100 grams of propellant.
The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are herein incorporated by reference.
In addition to a fuel constituent, pyrotechnic gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solid products must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate. Furthermore, many known gas generants produce solids that even in low concentrations, could be hazardous. Upon combustion, the use of components containing alkali and alkaline earth metals can result in the formation of highly alkaline reaction products. Compounds such as these could potentially cause severe caustic burns if contacted with the skin or eyes of a vehicle occupant.
While known nonazide gas generants provide operable amounts of gas with a minimum of solid combustion products, in many cases, the mass of gas generant required compared to the mass of gas produced is still cause for concern. The volume of the inflator necessarily reflects the volume of gas generant required to produce the gas needed to deploy the inflator. A reduction in the volume of gas generant needed, or an increase in the moles of gas produced per gram of gas generant, would result in a desirable reduction in inflator volume thereby enhancing design flexibility.
Yet another concern with known gas generant compositions is their compatibility with different materials used to form a pressure vessel in the gas inflator. Steel canisters are commonly used as the inflator pressure vessel in a passenger-restraint system because of the relatively high strength of steel at elevated temperatures. Given the emphasis on vehicle weight reduction, it is desirable that metals such as aluminum, and smaller or lighter steel vessels be utilized in the pressure vessel.
Engineering considerations require that vehicle operator restraint systems pass a "bonfire" test, wherein the inflator system is evaluated during exposure to fire. In the past, this has only been a concern for inflator canisters made of aluminum as the current steel pressure vessels routinely pass this test. Aluminum loses strength rapidly with increasing temperature, and may not be able to withstand the combination of increased external temperatures and excessive internal temperature and pressure generated upon combustion of the gas generant. An autoignition temperature of 175.degree. C. or less is considered sufficient for the safe use of aluminum canisters.
The inflator must be designed to maintain its structural integrity despite the high pressures produced by a rapidly burning gas generant. If the gas generant of the inflator can be made to autoignite at relatively low temperatures, for example, 150.degree. C. to 175.degree. C., then the pressure vessel can be made of a lightweight metal such as aluminum.